1. Field of the Invention
The present invention relates to a tire information detecting system for detecting tire information, including tire pressure, and, in particular, to cost-reducing means of the tire information detecting system by down-grading a tire information detecting unit composed of a microcomputer.
2. Description of the Related Art
A tire pressure monitoring system, which includes output means (tire information output unit) that outputs signals (tire information) including vibration frequency components of tires and detecting means (tire information detector) that retrieves resonance frequencies from the signal and then detects the tire pressures based on the resonance frequencies while a vehicle is being driven, has been proposed, as is disclosed in Japanese Unexamined Patent Application Publication No. 8-219920.
The output means is disposed in each tire of the vehicle and outputs sensed signals (tire information) including vibration frequency components of the tire to the detecting means composed of a wave shaping circuit, a ROM, and a RAM. The detecting means retrieves the resonance frequency of the tire from the signal including tire vibration frequency components and then detects the tire pressure based on the fact that, in normal vehicle use, a change in the resonance frequency is mostly generated by a change in a tire spring constant caused by a change in the tire pressure.
Consequently, according to the tire pressure monitoring system, an alarm that the tire pressures are below the allowed lower limit or the values of the tire pressures themselves can be displayed on a display unit of the vehicle so that a driver of the vehicle can check if the tire pressures are normal by monitoring the display. Therefore, driving safety of the vehicle can be increased.
Since a vehicle has four and more tires and output timings of signals from the output means, each mounted in each tire, are not synchronized, with reference to FIG. 7, a plurality of signals, four signals in FIG. 7, from the output means are sometimes input to the detecting means substantially at the same time.
Detecting means of such known tire pressure monitoring systems do not have a function that timely controls the reception processes of the signals in accordance with the output timing of the output means. Thus, the detecting means must process all the signals output from the output means. Consequently, the number of the reception processes performed in the detecting means is increased. In addition, as shown in FIG. 7, if signals from a plurality of the output means are input substantially at the same time, time intervals between the reception processes performed in the detecting means become shorter. Accordingly, the detecting means must include a high-performance microcomputer providing a high throughput and therefore the tire pressure monitoring system is disadvantageously costly.